Google's share of the U.S. search engine market has dropped below 65% for the first time in two years, new data shows.

Google, which has long been the dominant search engine in the U.S. and around the world, slipped from 65.1% of the U.S. market in July to 64.8% last month, reported comScore, an Internet traffic monitoring company.

According to comScore, Bing went from 14.4% of the U.S. search market in July to 14.7% in August. ComScore also reported that by the end of this year, Bing could double the 8% market share it started with when it entered the market back in 2009.

"Google hitting a two-year low in search share since 2009 and Bing almost doubling its share since 2009 doesn't seem like a coincidence," said Dan Olds, an analyst with The Gabriel Consulting Group. "Bing has definitely shown some staying power and steady growth in the two years since its introduction. We're seeing continued trench warfare in search with small shifts in share."

And Olds added that Microsoft has been pushing Bing hard in the market and its efforts are paying off.

"I think that Microsoft's aggressive promotion of Bing and tie-ins with other systems, software and Web providers is starting to pay off at least a bit," he said. "It's harder to explain why Yahoo search grew a tiny bit in this last survey - like everyone else, I expected their share to drop through the floor."

Both Google and Microsoft have been hitting hard at each other, trying to leap frog the other in terms of innovation.